The Stovebolt Site Forums The Shop Area Engine Shop Low compression on # 2 cylinder

Working on a 1956 235 engine that I purchased used. PO said it was rebuilt 14,000 miles ago. It had been non running for about 4 months since I got it. Currently set up on a run stand . It starts and runs ok.
A compression check on a warm engine reveals:
#1 145
#2 60
#3 142
#4 150
#5 150
#6 162

I readjusted all the valves and added one and one half oz oil thru the spark plug on #2. Compression increased to 90 but then next time returned to 60 psi. I did a leak-down test on #2 and it drops from 100 psi to 40 and holds there. Rockers on #2 valves were loosened to close them. Sounds as though air is escaping through breather/crankcase. I Wondering what the next step is to further diagnose this problem? If I need to pull the head and inspect the cylinder/piston I may decide to go ahead and install my older 235 engine in the truck and make this engine a separate project. Any suggestions or advice much appreciated.

I would think you have a cracked or broken compression ring. You could get yourself one of those el-cheapo endoscope (borescope) and check the cylinder wall for scoring. Those scopes are pretty handy, I use mine a lot more than I thought I ever would. Image quality is a bit sketchy but it gets the job done. Aside from that, you're gonna have to pull the head sooner or later.

Going to pull the manifolds and head. See what`s up. Hopefully minimal damage. Any recommendations on head gaskets? thanks for the reply.

I prefer the copper faced sandwich head gaskets. Perma-Torque gaskets were not made for the 235, and my luck with the common fiber gaskets hasn't been nearly as good as with the copper faced ones. Rock Auto is usually a good place to buy these. Good luck.

😉Jon G An old used car dealer I once worked for always had us emery cloth both head and block. Put it togrther dry ,re-torque after it got up to temperature. Dont think we ever lost a gasket that way.

Either use a borescope to check for a scored cylinder, or pull the head and oil pan and remove the piston from the low compression cylinder. You're probably dealing with a ring that was broken during the installation process. Was the engine bored during the "rebuild", or did it just get a new set of rings? A badly tapered cylinder can cause rings to lose their tension and compression sealing ability very quickly. There's a procedure in the tech tips section for checking cylinder taper with a piston ring, a feeler gauge, and a little work with a calculator. If the taper is much over .005" the rings aren't going to last very long.
Jerry

Ok I`m finally getting back to working on this engine. As suspected I have broken rings on #2. The condition of the cylinder walls don`t seem much different than the others that have good compression . No major scratches or gouges. Do I simply take my broken ring and piston to the local parts store and order a new set of rings? (sizing??) I measure the cylinder at the top at 3.602 Since this is my first effort into the internals of an engine I want to make sure I`m following good procedures prior to re-assembly . As this engine was purchased used and I don`t have documentation the previous owner said he purchased a running truck with rebuilt engine. Appreciate the help on next steps.

3.602" is .040" oversize for a 235 engine. It will be virtually impossible to buy a "single cylinder" ring set. 50 years ago, maybe- - - -now, no way! Pull the rest of the pistons and change all of the rings. If the quality of the job the #2 cylinder got is any indication, I would be very spooky about trusting the rest of the "overhaul" job the engine got. Be absolutely sure there is no ridge left at the top of the cylinders- - - -even a slight one will break a new ring in very short order. If there's no ridge, use a button hone to scuff up the cylinders well after wrapping the crankshaft journals with rags and masking tape to avoid contaminating the bearing surfaces. After honing, scrub the cylinders thoroughly with hot soapy water and a stiff bristle brush, then rinse repeatedly with VERY hot or boiling water, or use a pressure washer. Reassemble carefully, making sure you don't break any more rings!
Jerry

Ok ,l found 060 stamped on the piston top so I ordered a ring set for that size. I'm not familiar with a button hone but I have seen the 3 arm type that fits a drill. Not sure if that's the best one to use or if a special grit is needed. Appreciate the help so far. Ido try searching the previous posts for info but sometimes specific questions help along the way.

The 3 stone glaze breaker will work- - - -it's just somewhat obsolete technology. Be absolutely sure to get the cylinder walls CLEAN after honing- - - - -I completely wore out a set of rings on a 2500 mile road trip from California to Tennessee after a less-than-successful re-ring job. By the time we reached Nashville, the engine was using 3 quarts of oil to every tank of gas, and the corrugated oil ring expanders were rubbing the cylinder walls.

Here's what I've been using for many years- - - -kind of spendy, but it does a far better job of glaze breaking than the 3-finger hones. The 3 1/2" dimension will also work on a 235, which is 3 9/16" The next size bigger is 3 3/4", which would also work. These hones are lubricated with motor oil, and they make their own cross hatch pattern- - -no worries about the speed the hone is moved up and down.

https://www.ebay.com/itm/373829538892?

Jerry

“Button Hone”, “Dingle Ball Hone”, “Ball Hone”, all the same tool. Several hundred abrasive balls about 1/4 to 3/8 inch in diameter suspended on spring steel wires. Available at every Auto Parts Store. Much more efficient than a 3 stone Glaze Breaker. Be sure to keep whatever tool you use well lubricated as you move up and down in the cylinder.